Allow sliders to be centred

[RuboGubo]

Is your feature request related to a problem? Please describe.

You cant currently, I've tried using ui.vertical_centered() but it doesn't seem to work.

Describe the solution you'd like
make ui.vertical_centered() work

Describe alternatives you've considered
an option on the struct itself?

Additional context
n/a

[YgorSouza]

This is part of #4378. It's because the slider is made of multiple widgets and doesn't know how to calculate its size before drawing.

Currently you can center a slider without the numeric value and the text like this:

let spacing = &ui.style().spacing;
let size = [spacing.slider_width, spacing.slider_rail_height];
ui.vertical_centered(|ui| {
    ui.add_sized(
        size,
        egui::Slider::new(&mut self.value, 0..=120).show_value(false),
    );
});

To center a slider with text and numeric value, you have to calculate the size manually and include it in the add_sized call, or store the size on each frame to use it in the next frame, as discussed in #3468. #4631 is meant to improve this user experience.